That’s according to a survey by the Public Religion Research Institute and Religion News Service, which found that an additional 40 percent of people believe the narratives comprise a “theological story.”

In 2004, in contrast, two-thirds of Americans believed in the historical truth of the entire biblical Nativity accounts, according to a Newsweek poll.

“I don’t try and prove how right the Nativity” story is, said the Rev. Samuel Chambers Sr., pastor of Wayman Chapel African Methodist Episcopal Church in New Brighton, Pa. “God needs no proof. Either you believe in him or you don’t.” The Rev. James Farnan, pastor of St. Thomas More Catholic Church in Bethel Park, Pa., agreed.

“You can’t deny the historical nature of sacred Scripture,” he said. Noting that the gospels quote Hebrew Scripture passages they say were fulfilled in Jesus, Farnan added: “His is the only birth that has been predicted not only when and where but to what family and to what person.”

Other preachers say they don’t insist on belief in the details of the account but urge listeners to focus on what they see as the main message of passages — God becoming human to save humanity.

“The Christian faith is about way more than belief in the historical accuracy of every detail of the biblical story,” said the Rev. Roger Owens, professor of leadership and ministry at Pittsburgh Theological Seminary.

“You will be doing your congregation a great service if you move from the peripheral details to the center,” he said. “Belief in the virgin birth might be a late development, and it might not be attested to in every gospel, but if that’s what your faith stands on, it’s a flimsy faith. In Jesus, God took on flesh, and became truly human.”

Only four of the 89 chapters in the gospels talk about Jesus’ conception and birth, and yet the stories are among the best-known in all the Bible, celebrated in hymns from ancient times to the present, depicted in countless works of art and re-enacted in multitudes of pageants featuring kids in bathrobes portraying shepherds.

Christians have included Jesus’ virgin birth as part of their creeds since ancient times; in the 1920s, Protestant fundamentalists insisted that belief in the virgin birth be considered an essential of Christian faith; Roman Catholic dogma teaches that Mary remained a virgin her whole life and was herself conceived without sin.

Yet the growing skepticism comes amid two related trends.

One is the growing ranks of people claiming no religious affiliation — now approaching 20 percent. They are the most likely to doubt the Nativity accounts. At the same time, evangelical Protestants, who overwhelmingly affirm the historical accuracy of the accounts, are shrinking, said Robert Jones, chief executive officer of Public Religion Research Institute.

The other trend is the continued debate among Jesus scholars over how much of the gospels are historically accurate in every detail — and whether it’s necessary to believe that in order to believe Christian essentials. A recent best-seller, for example, Reza Aslan’s “Zealot,” disputes the notion that Jesus of Nazareth was born in Bethlehem.

No one writing about a messiah, said biblical scholar Ben Witherington III of Asbury Theological Seminary in Kentucky, would make up a story that would lead people to question the legitimacy of his birth — unless it were true.

“This is an honor-and-shame culture,” Witherington said. “We’re talking about a pregnancy out of wedlock in that culture, which could result in stoning. The idea that two gospel writers would independently make up a story about virginal conception is beyond credulity.”

The Rev. Dean Weaver, pastor of Memorial Park Church, a Presbyterian church in Hampton, Pa., said that while he affirms the historical truth of the narratives, he doesn’t use Christmas sermons to win over skeptics.

“Typically at Christmastime, it’s not about arguments or proof-texting or skepticism, it’s really about the beauty and the mystery of the Christmas narrative, and people entering into the fact that God came into the world because he loves us,” he said.

Plus, he said, “at Christmastime, you hope there’s some willingness to entertain the idea that the supernatural is actually real. Most people at this time of year, even if they’re skeptical, they’re open to at least wanting to believe that.”